Black oxide coated nitrided stainless steel product



Patented July 4, 1950 BLACK OXIDE COATED NIT-RIDED STAINLESS STEELPRODUGT Alexander L. Feild, Baltimore, Md., assignor, by

mesneassignments, to Armco Steel Corporation, a-corporation' of Ohio- N Drawing. Application February. 1-7, 1945, SerialNo. 578,554

This invention relates to' stainless steel, more particularly to corrosion-resistant case-hardened stainless steel.

One of theobjects of my invention-is to provide stainless steel products havingiag case-hardened finish in combination with outstanding resistance to corrosion.

A further object of my invention is the provision of nitrided stainless steel products having a protective. oxide surface film or coating which is thin, elasticand strong.

Other objects in part will be obvious and-in part will be pointed out hereinafter. v

The invention accordinglyconsists in the combination of elements and composition of materials, and in the relation ofJeachofthe-same to one or more of the others as describedherein, the scope .of the application of which, is indicated in the following claim.

Asv conducive to a clearer understanding of. certain features of my invention it may be noted that stainlesssteel is defined as an alloy steel which includes to 35% chromium, with. or without additional percentages of nickel, and with or without supplemental additions of manganese, silicon, cobalt, copper, molybdenum, tungsten, vanadium, columbium, titanium, sulphur, andthe like, for special purposes, and a balance which is substantially all iron.

It is a well accepted practice in the-prior art to nitride products of stainless steel, often in the machined condition; by subjectingthesame to treatment in a nitrogenousm'edium, suchas-in ammonia vapor as attemperatures-ranging between 930 F. and 1200 F., for increasing wear resistance and surface hardness of the metal'. The nitrided stainlessv steelz'products havebeen crease in corrosion :resistance which results from:-

the nitriding treatment, this .being true of products fashioned either from straight chromium or chromium-nickelgrades of stainlesssteel. Whilethe decrease in corrosion resistance .doesnotv place the steels in the same category of more easily.

corroded low-alloy carbonsteelsgthe outstanding durability offered. by stainless steels nevere theless is considerably,- diminishedi by. nitridingtreatment.

There is clearly an existing, incentive .for the development of stainless steel articles; 011 prod- IClitim. (Cl. 148-315) of time sufiicient to develop a corrosion-resiststainlessrsteel surfaces.

2 ucts so as to impart improved corrosion resistance to. the same and combine this characteristic with properties bestowed by a nitriding treatment. .An important consideration from a practical standpoint which frequently enters is to achieve the property of improved corrosion resistance without eilecting substantial changes in dimensions of nitrided stainless steel products which are of approximately finished or final size prior to treatment.

A nitrided stainless steel product surface or surface layer is rich in nitrides of chromium and includes a remarkably small amount of uncombined chromium as compared with non-nitrided This introduces an unusual problem in treating the nitrided surfaces especially considering that the beneficial properties gained through nitriding are to be maintained without substantial impairment. Among the possibilities of achieving improved corrosion resistance is treatment by chemical action, but such matters as effectiveness, economy and practicability are confronted in the provision of treating chemicals and operating conditions for obtaining the result desired.

I subject nitrided stainless steel products, illustratively wire, bars, rounds, strip and shapes which are more intricate as, for example, indicator needles, valves and valve parts, nozzles, cams, gears, drive wheels and pulleys, washers, bushings and bearings, to the chemical action of fused or molten oxidizing salt of such composition as to give a corrosion-resistant oxide surfacingon the nitrided metal products. The treatment just referred to is effected for a period ant. oxide coating or film on the nitrided productsexposed to the salt.

chemical action of fused salt rich in one or both of thefused-dichromates, I-achieve on the product a thin, elastic,,and5strongv black oxide surface filmwhich is instrumental in prolonging resistance of the nitrided metal to corrosive media. A satisfactory film of the character described is obtained by exposing the nitrided metal to dichromate salt treatment usually for from about 5 minutes to approximately 30 minutes time, particularly when sodium dichromate is the predominating constituent of the treating salt. On the other hand, when potassium dichromate as compared with sodium dichromate is included in the treating salt, a less active oxidizing action is exerted on the nitrided stainless steel. In certain instances, therefore, it may be desirable to extend the treating time up to 1 hour depending upon specific composition of the dichromate treating salt. Longer treating periods outside the range specified are not harmful or objectionable, but usually are unnecessary.

The usefulness of the oxide coating which I achieve is readily appreciated. The life of nitrided stainless steel products in corrosive atmospheres or in contact with corrosive media, such as steam, acids, salts, gases, fumes or oils, by virtue of the oxide coating is materially lengthened without substantially impairing hardness or resistance to wear and abrasion of the nitrided -case-on the steel. Provision of the black oxide coating ordinarily does not appreciably decrease serve as bearings, which revolve, rotate, or move back and forth, the oxide nitrided finish assists 1 in preventing a defect known as galling or seizing and givesimproved corrosion resistance as compared with uncoated nitrided articles. As a further example, oxide finished nitrided stainless the thickness of the nitrided case, as the thickness of this coating or film usually varies between 0.0001 inch and 0.000005 inch. The thin coating, therefore, has little effect on original overall dimensions of the nitrided products.

As illustrative, I provide a machined and'case treating temperature within the range of 615 F. 7

to 850 F. and preferably to a temperature between 730 F. and 800 F. The gear is immersed steel cams, gears, gear parts or other moving mechanical devices have the virtue of being highly resistant to wearand corrosion even in locations where grit, dust and dirt together with corrosive-gases, fumes or smoke are present. An interesting and still further example of oxide finishednitrided products which I provide are stainless steel heddle. wiresand similar stainless steel parts which are subjected to continuous bufling and abrading action of cloth threads and are exposed to corrosive acids, dyes and other chemicals carried by these threads.

The value of my corrosion-resistant oxide coated nitrided stainless steel products is refiected toan extent by a series of tests conducted on certain resulting products under various corrosive conditions. In brief, samples of several grades ofstainless, steelproducts were subjected to synthetic sea-water, running tap water, and to humid atmospheres for various lengths of time. These samples, consisting of black oxide coated nitrided specimens and uncoated nitrided specimens in pairs formed of like grades of stainless steel were compared at the end of the tests. The tests revealed that the nitrided but uncoated steels were more severely corroded than the oxide finished nitrided. specimens. Improvement in corrosion resistance: as a result of coating was indicated in each test. The analysis of some of the samples employed in the testsis noted for a period of about 20 minutes while the bath below:

Grade 0 Mn r s Si Cr. Ni Cb 12 Chrome 0.091 0.12 0.017 0.026 0.16 1) 0.128 0.34 0.018 0.022 0.23

D 0.064 1.25 0.0 2 0.015 0.60 25-20 Cr-NL- 0.125 1.40 0.011 0.019 0.23 17 Ohrome 0.078 0.44 0:016 0.038 0.43 18-8 Cr-NL 0.005 0.50 0.024 0.016 0.47 Do 0.062 2.10 0.023 0.012 0.55 25-20 Or-Ni 1. 02 0.013 0.38

I produce are particularly useful inmost if not all those commercial fields where nitrided and,

I havedescribed nitrided stainless steel articles and products of improved corrosion resistance. itwill be'understood that the invention also pertains 'to the improvement of carburized or other case hardened stainless steel articlesor products. Theresultant oxide finished case hardened prod uct also will be understood to form part of my invention.

As many possible embodiments may be made of myinvention and as many changes may be made in 'the'embodimen't's herelnbefore set forth, it

is understood that all matter described herein is to be interpreted as illustrative and not as uncoated stainless steels are desired; illustra-' a limitation.

REFERENCES CITED The following references are of record in the file of this patent:

6 7 UNITED STATES PATENTS Number Name 1 Date 1,930,388 Hatfield Oct. 10, 1933 2,189,785 Fahrney Feb. 13, 1940 2,343,418 Lampton et a] Mar. 7, 1944 2,394,899 Clingan Feb. 12, 1946 OTHER REFERENCES Tool Steels by Gill, Rose, Roberts, Johnstin, and George, page 489, publ. 1944, by Amer. Soc. for Metals, Cleveland, Ohio.

Steel, May 22, 1944, pages 80-81. 

